1. Technical Field
This invention relates in general to the field of electric converters, and more particularly, but not by way of limitation to a programmable electric converter.
2. Background
Electro-magnetic machines have been used as generators or as motors or both simultaneously. The operation of the electro-magnetic machine is determined by the type of energy used to drive the machine and the type of energy which is obtained from the operation of the machine. For example, if electrical energy is delivered to the machine and mechanical energy is removed from the machine, then the machine will operate as a motor. Likewise, if mechanical energy is delivered to the machine and electrical energy is removed from the machine, then the machine will act as a generator. In some cases the machine may act both as a motor and as a generator, such as by delivering electrical energy to the machine and removing both electrical energy and mechanical energy therefrom.
In general, electro-magnetic machines usually comprise a rotor and stator, with one or both of such components having electrically induced magnetic poles. The magnetic flux lines emanating from the magnetic poles serve either to energize the rotational movement or to induce an electrical current in conductors provided adjacent thereto. Such electro-magnetic devices include generally stationary and C-shaped magnets which are arranged about the circumference of a circle and having a plurality of coils arranged around the circumference of a circle which communicates through the openings in the C-shaped magnets.
If mechanical energy, such as an external torque force, is applied to the central shaft for rotating the coils through the permanent magnets, then the machine operates as a generator. When operated in a generator mode, the external torque source forces rotation of the shaft (and thus the rotor and the magnets), and the interaction of the magnets and the windings causes a magnetic flux to loop the windings in the slots. As the rotor rotates, the magnetic flux in the stator structure changes, and this changing flux results in generation of voltage in the windings, which results in an output current that can be used to power electrical devices, or be stored for later use. When operated in a motor mode, a voltage from an external source is applied to the stator windings, which causes current flow in the windings and results in a magnetic flux to be set up in the magnetic circuit formed by the teeth and back iron. When current is supplied in an appropriate manner to the windings, the rotor can be made to rotate and thus produce usable torque. The operation of such machines is thus well understood.
Prior art electro-magnetic machines suffer from a variety of limitations which have limited their usefulness somewhat. For example, the frequency and voltage of a permanent magnet electro-magnetic machine operating as a generator may only be varied by varying the rotor speed, which limits the usefulness of such a generator in circumstances where the rotor rotation speed cannot be independently controlled.
Commutator-type motors do not operate well on high-frequency AC because the rapid changes of current are opposed by the inductance of the motor field. Although commutator-type universal motors are common in 50 Hz and 60 Hz household appliances, they are often small motors, less than 1 kW. The induction motor was found to work well on frequencies around 50 to 60 Hz but not as well at a frequency of, say, 133 Hz. There is a fixed relationship between the number of magnetic poles in the induction motor field, the frequency of the alternating current, and the rotation speed; so, a given standard speed limits the choice of frequency (and the reverse).
Generators operated by slow-speed reciprocating engines will produce lower frequencies, for a given number of poles, than those operated by, for example, a high-speed steam turbine. For very slow prime mover speeds, it would be costly to build a generator with enough poles to provide a high AC frequency. As well, synchronizing two generators to the same speed was found to be easier at lower speeds. While belt drives were common as a way to increase speed of slow engines, in very large ratings (thousands of kilowatts) these were expensive, inefficient and unreliable. The steadier rotation speed of high-speed machines allowed for satisfactory operation of commutators in rotary converters. The synchronous speed N in RPM is calculated using the formula,
  N  =            120      ⁢      f        P  
where f is the frequency in Hertz and P is the number of poles.
It would therefore be desirable to improve the controllability of electro-magnetic machines, generally. Accordingly, there is a need to provide an improved electro-magnetic machine which addresses these and other limitations of the prior art.